Especially because of environmental issues, new energy sources, more environmentally friendly and, for example, having a better efficiency than the above-mentioned energy sources, have been developed. Fuel cells, by which energy of fuel, for example biogas, is directly converted to electricity via a chemical reaction in an environmentally friendly process and electrolysers, in which electricity is converted to a fuel, are promising future energy conversion devices.
Renewable energy production methods, such as photovoltaic and wind power, face issues in seasonal production variations as their electricity production is limited by environmental effects. In the case of over production, hydrogen production through water electrolysis is suggested to be one of the future energy storing options. Furthermore, an electrolysis cell can also be utilized to produce high quality methane gas from renewably biogas stores.
A fuel cell, as presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with fuel 108 producing electrons, water and also typically carbon dioxide (CO2). Anode 100 and cathode 102 are connected through an external electric circuit 111 having a load 110 for the fuel cell withdrawing electrical energy alongside heat out of the system. The fuel cell reactions in the case of methane, carbon monoxide and hydrogen fuel are shown below:Anode: CH4+H2O=CO+3H2 CO+H2O=CO2+H2 H2+O2−=H2O+2e−Cathode: O2+4e−=2O2−Net Reactions: CH4+2O2=CO2+2H2OCO+1/2O2=CO2 H2+1/2O2=H2O
In an electrolysis operating mode (solid oxide electrolysis cells (SOEC)) the reaction is reversed, i.e. heat, as well as electrical energy from a source 110, are supplied to the cell where water and often also carbon dioxide are reduced in the anode side forming oxygen ions, which move through the electrolyte material to the cathode side where de-ionization to oxygen takes place. It is possible to use the same solid electrolyte cell in both SOFC and SOEC modes. In such a case and in the context of this description the electrodes are typically named anode and cathode based on the fuel cell operating mode, whereas in purely SOEC applications the oxygen electrode may be named as the anode, and the reactant electrode as the cathode.
Solid oxide electrolyser cells operate at temperatures which allow high temperature electrolysis reaction to take place, said temperatures being typically between 500-1000° C., but even over 1000° C. temperatures may be useful. These operating temperatures are similar to those conditions of the SOFCs. The net cell reaction produces hydrogen and oxygen gases. The reactions for one mole of water are shown below, with reduction of water occurring at the anode:Anode: H2O+2e−--->2H2+O2−Cathode: O2−--->1/2O2+2e−Net Reaction: H2O --->H2+1/2O2.
In Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolyzer (SOE) stacks the flow direction of the cathode gas relative to the anode gas internally in each cell as well as the flow directions of the gases between adjacent cells, are combined through different cell layers of the stack. Further, the cathode gas or the anode gas or both can pass through more than one cell before it is exhausted and a plurality of gas streams can be split or merged after passing a primary cell and before passing a secondary cell. These combinations serve to increase the current density and minimize the thermal gradients across the cells and the whole stack.
A SOFC delivers in normal operation a voltage of approximately 0.8V. To increase the total voltage output, the fuel cells can be assembled in stacks in which the fuel cells are electrically connected via flow field plates (also: interconnector plates, bipolar plates). The desired level of voltage determines the number of cells needed.
Bipolar plates separate the anode and cathode sides of adjacent cell units and at the same time enable electron conduction between anode and cathode. Interconnects, or bipolar plates are normally provided with a plurality of channels for the passage of fuel gas on one side of an interconnect plate and oxidant gas on the other side. The flow direction of the fuel gas is defined as the substantial direction from the fuel inlet portion to the fuel outlet portion of a cell unit. Likewise, the flow direction of the oxidant gas, the cathode gas, is defined as the substantial direction from the cathode inlet portion to the cathode outlet portion of a cell unit.
The cells can be stacked one on top of each other with a complete overlap resulting in a stack with for instance co-flow having all fuel and oxidant inlets on one side of the stack and all fuel and oxidant outlets on the opposite side. One feature affecting the temperatures of the structure in operation is steam reformation of the fuel that is fed into the cell. Steam reformation is an endothermic reaction and cools the fuel inlet edge of the cell.
Due to the exothermicity of the electrochemical process, the outlet gases leave at higher temperature than the inlet temperature. When endothermic and exothermic reactions are combined in an SOFC stack a significant temperature gradient across the stack is generated. Large thermal gradients induce thermal stresses in the stack which are highly undesirable and they entail differences in current density and electrical resistance. Therefore the issue of thermal management of an SOFC stack exists: to reduce thermal gradients enough to avoid unacceptable stresses and to maximize electric efficiency through homogenous current density profile.
Known fuel cells or electrolyzer cells involve thermal gradients due to uneven gas distribution over the electrolyte element. This causes lower duty ratio of the cell and thermal stresses due to uneven thermal and operational load also deteriorates the cell.